On a couple of pages I have, each page uses an include file that writes a couple of session variables.

Each page has its own post form. It's a "signup" site.

Everything goes well until I click the Back button at some point.

Then I get this message in IE5 (not a PHP error):

"Warning: Page has Expired The page you requested was created using information you submitted in a form. This page is no longer available. As a security precaution, Internet Explorer does not automatically resubmit your information for you.